There is known a field emission device applicable to various types of equipment such as X-ray apparatus, electron tube, lighting apparatus etc., in which an emitter (an electron source made of a carbon material or the like) and a target are opposed to each other (with a predetermined distance left therebetween) in a vacuum chamber of a vacuum vessel such that field emission from the emitter is caused (electrons are generated and emitted) with the application of a voltage between the emitter and the target so as to thereby emit an electron beam onto the target and perform a desired function (e.g. a radioscopic resolution function due to external X-ray radiation in the case of X-ray apparatus).
Further, techniques have been studied to suppress scattering of the electron beam from the emitter by adopting a triode structure where a grid electrode is interposed between the emitter and the target, by forming a curved surface on an electron generating portion of the emitter (which is located opposite the target and from which electrons are generated), or by arranging a guard electrode of the same potential as the emitter on a circumferential edge side of the emitter (see Patent Documents 1 and 2).
It is preferable to emit the electron beam by generating electrons only from the electron generating portion of the emitter with the application of the voltage as mentioned above. In the presence of an unnecessary minute projection or dirt in the vacuum chamber, however, it becomes likely that a flashover phenomenon will occur. The field emission device may thus not attain a desired withstand voltage.
The above problem arises in the case where the guard electrode or the other component inside the vacuum chamber (more specifically, the target, the grid electrode, the guard electrode or the like; hereinafter simply referred to as the “guard electrode etc.” as required) has a part that can easily cause local field concentration (e.g. a minute projection is formed on the guard electrode etc. by processing work), in the case where a gas component is adsorbed on the guard electrode etc. and in the case where the guard electrode etc. contains an element that can easily generate electrons. In these cases, the amount of electrons generated in the field emission device becomes unstable due to the formation of an electron generating portion on the guard electrode etc. As a result, it becomes likely that the electron beam will be scattered. This leads to the problem of, in the case of X-ray apparatus, X-ray defocusing.
It has accordingly been studied, as a technique for suppressing the flashover phenomenon (i.e. a technique for stabilizing the electron generation amount), to perform voltage discharge conditioning treatment (reforming (regeneration); hereinafter referred to as “reforming treatment”) of applying a voltage (high voltage etc.) to the guard electrode etc. (e.g. the guard electrode and the grid electrode) and repeatedly effecting discharge on the guard electrode etc.